This invention relates generally to methods for making a collimator used in an imaging system, and to the collimator made from these methods.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile of the object.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Detector elements are configured to perform optimally when impinged by x-rays travelling a straight path from the x-ray source to the detector elements. Particularly, detector elements typically include scintillation crystals which generate light events when impinged by an x-ray beam. These light events are output from each detector element and directed to photoelectrically responsive materials in order to produce an electrical signal representative of the attenuated beam radiation received at the detector element. Typically, the light events are output to photomultipliers or photodiodes which produce individual analog outputs. Detector elements thus output a strong signal in response to impact by a straight path x-ray beam.
X-rays often scatter when passing through the object being imaged. Particularly, the object often causes some, but not all, x-rays to deviate from the straight path between the x-ray source and the detector. Therefore, detector elements are often impinged by x-ray beams at varying angles. System performance is degraded when detector elements are impinged by these scattered x-rays. When a detector element is subjected to multiple x-rays at varying angles, the scintillation crystal generates multiple light events. The light events corresponding to the scattered x-rays generate noise in the scintillation crystal output, and thus cause artifacts in the resulting image of the object.
To reduce the effects of scattered x-rays, scatter collimators are often disposed between the object of interest and the detector array. Such collimators are constructed of x-ray absorbent material and positioned so that scattered x-rays are substantially absorbed before impinging upon the detector array. For one known collimator, it is important that the scatter collimator be properly aligned with both the x-ray source and the detector elements so that only x-rays travelling on a substantially straight path impinge on the detector elements.
Known collimators are complicated and cumbersome to construct. In addition, it is difficult to satisfactorily align known collimators with the x-ray source and the detector elements to both absorb scattered x-rays and shield sensitive portions of the detector elements.